1. Field
The present invention relates to a sewing machine, and more particularly to a thread tension device therefor including a thread tension dial which is manually turned by an operator so that a tension applied to a needle thread clamped between a pair of thread tension discs is adjusted.
2. Related Art
Conventional lockstitch sewing machines are provided with a thread tension device in a sewing arm. The thread tension device includes a thread tension dial for adjusting the strength of tension applied to a needle thread. The thread tension dial has an upper end partially exposed outward from an upper part of the sewing arm, so as to be externally operable. When workpiece cloth is to be sewn, an operator manually turns the thread tension dial according to a thickness and type of the workpiece cloth and a type of the sewing thread. As a result, the operator can adjust the strength of tension applied to the needle thread.
For example, a publication JP-A-H06-134165 discloses a threading error proofing device for a sewing machine. A vertical display panel is mounted on an upper front interior of the sewing arm. A needle thread tension mechanism is provided right behind the display panel. The thread tension mechanism comprises needle thread tension discs, a tension release bar, a thread tension adjusting member, a needle thread tension spring and a spring seat. The tension release bar is provided for opening and closing the thread tension discs. The needle thread tension spring is provided between the needle thread tension discs and the thread tension adjusting member to bias the needle thread tension discs in a closing direction. The spring seat receives an end of the needle thread tension spring.
The needle thread tension mechanism is thus disposed right behind an exterior cover inside the sewing arm. When a test operation of a sewing machine is carried out after assembly by a sewing machine manufacturer, tension applied to a needle thread is measured by setting the thread tension dial to a reference scale. In this case, when a measured value of tension applied to the needle thread differs from a reference adjustment value, the operator changes a magnitude of spring force of the needle thread tension spring, thereby setting the tension of the needle thread to the reference adjustment value. In this case, the operator needs to detach the exterior cover of the sewing arm and then manually turn the thread tension adjusting member receiving the right end of the needle thread tension spring using an adjustment tool.
Thus, the operator needs to detach the exterior cover of the sewing arm and then turn the thread tension adjusting member using the tool when the magnitude of tension applied to the needle thread in the test operation differs from the reference adjustment value in the threading error proofing device described in the above-noted publication. As a result, detachment of the exterior cover is troublesome. Moreover, the thread tension adjusting member needs to be manually turned, whereupon a troublesome adjusting operation is necessitated. As a result, there is a problem that the adjusting work necessitates much time.